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Don't Wait Too Late To Use Soil Nitrate Test For CornDon't Wait Too Late To Use Soil Nitrate Test For Corn

Iowa farmers reminded to scout for insects, sample soil for late spring soil nitrate test, apply postemergence herbicides in timely manner.

Rod Swoboda 1

June 2, 2014

7 Min Read

Each Monday Iowa State University Extension field agronomists from around the state hold a conference call with agronomists on campus to provide an update on crop issues and conditions.

As the calendar enters June, Iowa farmers are reminded to continue to scout corn for black cutworm until corn reaches V5 growth state (5 leaf collars). There have been some fields with cutworms reported in southeast, central and south central Iowa. For more information, click here.


Weed control issues, postemergence herbicide application
"I've noticed waterhemp and other weeds coming in several of the fields I've been in," says Mark Johnson, an ISU Extension field agronomist in central Iowa. "It's best to spray for waterhemp soon, as it is much easier to control before it is 4 inches tall. Also, the later your spray, which is true for many herbicides, such as Flexstar soybean herbicide and others, the more chance of carryover problems when the field is planted to corn next year. With later soybean planting this year, you need to keep the 10 month crop rotation interval in mind for next year's corn."

Another weed control issue -- the potential for interactions between post herbicides and soil insecticides exists. Check both labels -- insecticide and herbicide. The biggest concern is with Counter insecticide, says Johnson. The same plant enzyme used to metabolize the insecticide is used to metabolize some herbicides, in this case ALS herbicides. Using both pesticides in the same crop year can overload the plant metabolic processes."

Herbicide carryover injury symptoms showing up on corn
Some farmers have seen herbicide carryover injury symptoms on corn this spring, caused by soybean herbicide applied in the field a year ago. Virgil Schmitt, Iowa State University Extension field agronomist in southeast Iowa, says this carryover is caused by fomesafen, the active ingredient in Flexstar and Reflex soybean herbicides as well as several "generics," such as Battlestar, Dawn, Rhythm, Ringside, Rumble, Shafen, Topgun, etc. Fomesafen is also present in combination with other active ingredients in several products, such as Flexstar GT, Flexstar GT 3.5, Marvel, Prefix and Statement. Fomesafen was applied to many soybean fields in 2013.


"The Flexstar label states that corn should not be planted for 10 months after Flexstar application, and in many fields this spring, corn was planted less than 10 months after the fomesafen application," says Schmitt. "In addition, the dry weather in 2013 slowed down the normal rate of chemical breakdown. The cold winter may also have reduced the amount of breakdown that would normally occur."

Corn injured by fomesafen carryover will exhibit veinal chlorosis. The veins will be chlorotic while the area of the leaf between the veins will remain green. Some people are getting this symptom confused with sulfur deficiency, notes Schmitt. Sulfur deficiency is the exact opposite, with the veins remaining green and the interveinal material becoming chlorotic or yellow. This is often seen in low organic matter areas without a history of animal manure and in instances where rapid top growth outpaces root development and the ability to move sulfur into the plant rapidly enough.

"Corn normally recovers quickly from fomesafen injury," says Schmitt. "I'm not aware of any instance where yield loss due to this injury has been documented."

It's time for the late spring soil nitrate test
The earliest planted corn is approaching the time to check the N status of the soil by taking 1-foot depth soil samples when the corn is 6 to 12 inches tall. Schmitt says at least 16 soil cores (24 is better) should go into each sample and about a cup of this (soil bag full) is what you send to a lab for analysis. Cores should be pulled in a systematic way going across corn rows (i.e. first core pulled in the row, next one is 1/8th the distance between rows, next is 1/4th the distance between rows, etc.).

For more information on the process is in Extension publication PM1714.   An information sheet for sending samples to ISU is available here. The cost for analysis is $5 per soil sample.


Do you have a poor stand of corn or soybeans?
It would take a very poor stand of corn for you to justify replanting it at this late of a date, says ISU's Mark Johnson. "In the area I cover in central Iowa the corn is virtually all planted and the vast majority of the soybeans are in the ground," he noted on June 2. "It is late enough now that it would take a very poor corn stand to justify replanting." He provides this table based on ISU research which shows the yield potential at various planting dates.

                                                                     Planting Date

Population (Plants/Acre)

April 20-May 5

May 5-May 15

May 15-May 25

May 25-June 5

June 5-June 15

                                                  Percent Maximum Yield
























































This table shows the optimum planting date range yield equals 100%. Example: The field was originally planted May 10 and has a poor stand. To decide if it would pay to replant, look to the column with the proposed replanting date. If replanting June 4, you could hope for 70% of optimum. Look at the column with the original planting date of May 10. The original stand would have to be less than 11,000 to make replanting worthwhile and even less than that, when you consider replanting expenses.

"For the most part, the corn is finally starting to even out, green up and look decent," says Clarke McGrath, ISU Extension field agronomist in western Iowa. "While it has endured emergence issues, seedling diseases, frost damage and even some stand loss to hail in a few areas for the most part it now looks like most of the stands were worth keeping and the young plants are starting to develop their nodal or permanent root system. We've had few reports of significant insect issues so far. Postemerge spraying is going according to schedule, rigs running hard in small windows of opportunity between muddy fields and windy afternoons."

If you are considering replanting soybean, here is a good article to consult.

About the Author(s)

Rod Swoboda 1

Editor, Wallaces Farmer

Rod, who has been a member of the editorial staff of Wallaces Farmer magazine since 1976, was appointed editor of the magazine in April 2003. He is widely recognized around the state, especially for his articles on crop production and soil conservation topics, and has won several writing awards, in addition to honors from farm, commodity and conservation organizations.

"As only the tenth person to hold the position of Wallaces Farmer editor in the past 100 years, I take seriously my responsibility to provide readers with timely articles useful to them in their farming operations," Rod says.

Raised on a farm that is still owned and operated by his family, Rod enjoys writing and interviewing farmers and others involved in agriculture, as well as planning and editing the magazine. You can also find Rod at other Farm Progress Company activities where he has responsibilities associated with the magazine, including hosting the Farm Progress Show, Farm Progress Hay Expo and the Iowa Master Farmer program.

A University of Illinois grad with a Bachelors of Science degree in agriculture (ag journalism major), Rod joined Wallaces Farmer after working several years in Washington D.C. as a writer for Farm Business Incorporated.

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